Technical Field
This disclosure relates to polymer electrolyte materials, such as those used in proton exchange membrane (PEM) fuel cells.
Description of the Related Art
Fuel cells are commonly used for generating electric current. A single fuel cell typically includes an anode electrode, a cathode electrode, and an electrolyte between the anode and cathode electrodes for generating an electric current in a known electrochemical reaction between a fuel and an oxidant. The electrolyte may be a polymer membrane, which is also known as a proton exchange membrane or “PEM.”
One common type of polymer exchange membranes is per-fluorinated sulfonic acid (PFSA) polymer membrane, such as NAFION® (E. I. du Pont de Nemours and Company). PFSA polymer consists of a perfluorinated carbon-carbon backbone, to which are attached perfluorinated side chains. Each side chain terminates in a sulfonic acid group that works as a proton exchange site to transfer or conduct protons between the anode and cathode electrodes.
The proton conductivity of PFSA polymers varies in relation to relative humidity (RH) and temperature. The relation between conductivity and level of hydration is based on two different mechanisms of proton transport. One is the vehicular mechanism, where the proton transport is assisted by the water in the membrane, and the other is the hopping mechanism, where the proton hops along the sulfonic acid sites. While vehicular mechanism is dominant at high relative humidity conditions, the hopping mechanism becomes important at low relative humidity conditions.
PEM fuel cells, especially for automobile applications, are required to be able to operate at high temperature (≧100° C.) and low RH (≦25% RH) conditions, in order to reduce the radiator size, simplify the system construction and improve overall system efficiency. Consequently, PEM materials with high proton conductivity at high temperature and low RH conditions are needed.
PFSA polymer is usually prepared by free radical copolymerization of tetrafluoroethylene (TFE) and per-fluorinated (per-F) vinyl ether monomer (such as perfluoro-2-(2-fluorosulfonylethoxy) propyl vinyl ether, or “PSEPVE”, for Nafion®). One approach to produce a PFSA polymer with improved proton conductivity is to decrease the TFE content in the product polymer. An indicator of conductivity of an electrolyte material is equivalent weight (EW), or grams of polymer required to neutralize 1 mol of base. The most common equivalent weights of commercially available PFSA polymers (such as NAFION®) are between ˜800 and ˜1100 g/mol, which provide a balance between conductivity and mechanical properties. While PFSA polymer with EW in this range is needed, increasing conductivity below a certain EW threshold, saying ˜700 g/mol, renders the electrolyte water soluble and not suitable for PEM applications.
Per-F sulfonimide (SI) acids (such as Bis (trifluoromethane) sulfonimide, CF3—SO2—NH—SO2—CF3) show favorable properties, including strong acidity, excellent chemical and electrochemical stability, for PEM fuel cell applications. Linear per-F sulfonimide polymers (PFSI), prepared by copolymerization of TFE and SI-containing per-F vinyl ether monomer, were first reported by DesMarteau, et al. (U.S. Pat. No. 5,463,005). Such type of linear PFSI polymers with the EW in the range of 1175-1261 g/mol for PEM application was reported by Creager, et at (Polymeric materials: science and engineering—WASHINGTON—80, 1999: 600). Per-F vinyl ether monomer that contains two SI groups was also synthesized, and the corresponding linear PFSI polymer with the EW of 1175 g/mol was prepared and demonstrated to have high thermal and chemical stability in PEM fuel cell operating conditions (Zhou, Ph.D. thesis 2002, Clemson University). Reducing TFE content in the PFSI polymers is an efficient way to increase the proton conductivity of the product polymers. Linear PFSI polymer with the EW of 970 g/mol was reported in the literature (Xue, thesis 1996, Clemson University). However, such type of linear PFSI polymers with even lower EW is difficult to synthesis through free-radical copolymerization process and also renders the polymer water soluble below a certain EW threshold.
The preparation of PFSI polymer with calculated EW of ˜1040 by chemical modification of PFSA polymer resin (in —SO2—F form) was reported in a Japanese patent (Publication No: 2002212234). Furthermore, a more efficient chemical modification process was reported by Hamrock et al. (Publication No. WO 2011/129967). In this process, a linear PFSA polymer resin (in —SO2—F form) was treated with ammonia in acetonitrile (ACN) to convert the —SO2—F groups to sulfonamide (—SO2—NH2) groups, which then reacted with a per-F disulfonyl difluoride compound (such as F—SO2—(CF2)3—SO2—F) to convert to —SO2—NH—SO2—(CF2)3—SO3H in the final product. By starting with a 3M's PFSA (in —SO2—F form) with EW of ˜800 g/mol, water-insoluble polymer electrolyte with EW as low as ˜625 g/mol was reported. However, polymer electrolyte with even lower EW (<625 g/mol) resulted in a water soluble polymer and hence is not suitable for PEM applications.
Cross-linking is known as an effective strategy to prevent polymers from being soluble in water and organic solvents. This step is known to improve polymers' mechanical strength. Cross-linking PFSA polymer (in —SO2—F form) can be achieved by a coupling reaction of a sulfonyl fluoride (—SO2—F) group and a sulfonamide (NH2—SO2—) group to form a sulfonimide acid (—SO2—NH—SO2—). The resulting sulfonimide group also works as a proton conducting site.
Uematsu et al. (Journal of Fluorine Chemistry 127 (2006) 1087-1095) reported using thermal treatment (270° C.) to couple sulfonyl fluoride groups and sulfonamide groups in terpolymers of TFE, PSEPVE and sulfonamide-containing per-F vinyl ether monomer to form sulfonimide groups as cross-linking sites in the polymer matrix. An improvement in mechanical strength of polymer matrix was shown, without reduction in equivalent weight.
Hamrock et al. (US2009/041614, US2006/0160958, US2005/0113528, U.S. Pat. No. 7,060,756, EP1690314) proposed to use aromatic cross-linking agents to react with PFSA polymer (in —SO2—F and/or —SO2—Cl form) to generate aromatic sulfone-containing cross-links in the polymer matrix. The proposed reaction conditions include thermal treatment at high temperature (160° C. or higher) and with a Lewis acid as catalyst. The proposed product polymer may have EW lower than 900 g/mol. The even lower EW (≦700 g/mol) cross-linked polymer products were not mentioned in these patents. In addition, the introduction of aromatic ring structures into the polymer matrix compromised chemical stability and could lead to inferior durability of product polymer membranes in highly acidic and highly oxidizing conditions in PEM fuel cells.